Growing global concern of chemical air pollutants has led a number of governing bodies to increase the emissions standards for exhaust from combustion engines. In particular, emissions standards are increasing for emissions from diesel engines and diesel vehicles, such as heavy-duty diesel engines and heavy-duty diesel vehicles. The United States Environmental Protection Agency (“U.S. EPA”) defines a “heavy-duty vehicle” as those vehicles with a gross vehicle weight rating of more 8,500 pounds, except certain passenger vehicles weighing less than 10,000 pounds. The U.S. EPA further defines a “light heavy-duty diesel engine” as an engine used in a vehicle heavier than 8,500 pounds but lighter than 19,500 pounds, with the exception of certain passenger vehicles weighing less than 10,000 pounds. The U.S. EPA further defines a “medium heavy-duty diesel engine” as an engine used in a vehicle 19,500 pounds or heavier but 33,000 pounds or lighter. The U.S. EPA further defines a “heavy heavy-duty diesel engine” as an engine used in a vehicle more than 33,000 pounds. In California, “light heavy-duty diesel engines” are defined as engines used in a vehicle heavier than 14,000 pounds but lighter than 19,500 for those vehicles manufactured in the year 1995 or later. In Europe, a “heavy-duty diesel engine” has been considered to be an engine used in a vehicle of more than 3.5 metric tons (more than 7,716 pounds). A heavy-duty diesel vehicle may therefore be considered to be a diesel vehicle with a weight of more than about 7,700 pounds, or more than about 8,500 pounds, or more than about 10,000 pounds, or more than about 14,000 pounds, or more than about 19,500 pounds, or more than about 33,000 pounds, and a heavy-duty diesel engine is an engine used in a heavy-duty diesel vehicle.
Conventional, commercially available catalytic converters generally use platinum group metal (PGM) catalysts deposited on substrates using only wet-chemistry methods, such as precipitation of platinum ions and/or palladium ions from solution onto a substrate. These PGM catalysts are a considerable portion of the cost of catalytic converters. Accordingly, any reduction in the amount of PGM catalysts used to produce a catalytic converter is desirable. Commercially available catalytic converters also display a phenomenon known as “aging,” in which they become less effective over time due, in part, to an agglomeration of the PGM catalyst, resulting in a decreased surface area. Accordingly, reduction of the aging effect is also desirable to prolong the efficacy of the catalytic converter for controlling emissions.
Unlike light-duty vehicles, where much of the developing technology in catalytic converters concerns increasing the catalytic efficiency of “cold-start” emissions wherein the catalytic converter is cool for much of the running time, heavy-duty vehicle engines are on for longer periods of time, increasing the average running temperature of the catalytic converter. Furthermore, emissions standards often differ for light-duty and heavy-duty vehicles. Nitrogen oxide (NOx) emissions are of a particular concern, however technologies must be developed to meet other increasing emissions standards as well, including a reduction of pollutants such as carbon monoxide (CO) and hydrocarbons (HC). At the elevated average running temperature of heavy-duty vehicle catalytic converters, however, catalysis of NOx emissions and CO or HC emissions are not always maximally efficient when using the same catalyst. Therefore, there remains a need to develop catalytic converter washcoats, and catalytic converters using those washcoats, that are able to meet the increasing emissions standards for heavy-duty vehicle combustion engines while reducing the effect of aging and minimizing the amount of PGM required to produce the catalytic converter.